1. Field of the Invention
The present invention relates to an image reading apparatus such as a copying machine.
2. Description of the Related Art
Hitherto, image reading apparatuses using a charge-coupled device (CCD) line sensor for scanning and reading a document placed on a glass table have been known. Some of these image reading apparatuses have a function of automatically detecting the size of a document placed on the glass table. Also, the size of paper to be copied and the scale are automatically set.
A variety of methods can be used in order to detect the size of a document. According to one of the methods, a reflective sensor for detecting the size of a document is placed at a specific position in the main and sub scanning directions under the glass table. Then, the existence of the document at the specific position is detected by the sensor so as to determine the size of the document based on the detection result. According to another method, when a command of reading a document is output, the whole document area is scanned and read (pre-scan) before reading, and the size of the document is detected based on the output image signal.
In the method using only the reflective sensor, many sensors corresponding to various sizes of a document that can be placed on the glass table are necessary, and thus the configuration becomes complicated and the cost increases. On the other hand, in the method using a pre-scan, the pre-scan must be performed every time a document is placed on the glass table. Therefore, it takes time from when a command for starting reading is output until image reading is completed.
Accordingly, a method in which the above-described two methods are combined has been proposed. In this method, an open/closed state of a cover for covering a glass table is recognized and image data when the cover is half-closed is compared with image data when the cover is completely closed. Accordingly, the size in the main scanning direction is detected, and the size in the sub scanning direction is detected by using the known reflective sensor.
In this method, when the cover is half-closed, the area other than the document area does not reflect the light from a light source, and is read as black. On the other hand, when the cover is completely closed, the area other than the document area is read as white (when the rear side of the cover is white). Thus, by comparing the image data, the document area can be recognized and thus the size of the document can be detected.
In this method, only a reflective sensor for sub scanning direction is required and pre-scan need not to be performed, and thus time for reading the document is shortened. In the above-described method, however, data read by a CCD is used in order to detect the size in the main scanning direction. Accordingly, the CCD must be driven during a standby mode in which reading is not performed so as to read a document.
In the above-described method of detecting the size of a document, the CCD is always driven even when reading is not performed, and thus electrical power is wasted. Also, the temperature of the CCD rises when the CCD is driven. In a color sensor, in particular, temperature degradation in a color filter may be caused. The degradation in a color filter results in variation in the spectral characteristic, and may have a bad effect on a read color image. Also, a fan may be provided in the apparatus in order to suppress rise in the temperature of the CCD. In this case, however, vibration caused by rotation of the fan badly affects the image, for example, color drift is caused. Further, the cost increases.